FIG. 1 shows a flyback converter having a synchronous rectification function according to the prior art. As shown in FIG. 1, a flyback transformer TF has a primary winding Np11 having one end connected to a rectified input DC voltage Vin and another end connected to a drain terminal of a main switch MS which is implemented by a MOS transistor. A primary switch circuit 11 is connected to a gate terminal of the main switch MS and configured to issue pulse signals to control the switching operations of the main switch MS. When the main switch MS is turned on, the primary winding Np11 receives an input current I1 from an input terminal and thus stores energy in the primary winding Np11. In the meantime, a synchronous switch SS that is implemented by a MOS transistor and connected to a second secondary winding Ns12 of the flyback transformer TF is turned off through a secondary synchronous rectification driver 13 which is connected to a first secondary winding Ns11. Next, when the main switch MS is turned off, the synchronous switch SS is turned on through the secondary synchronous rectification driver 13 so that the energy stored in the primary winding Np11 is transferred to the secondary side of the flyback transformer TF, thereby inducing a high-frequency AC voltage across the second secondary winding Ns12 and causing a secondary current to flow through the synchronous switch SS. Here, the secondary synchronous rectification driver 13 is driven by the driving voltage provided by the first secondary winding Ns11, and the first secondary winding Ns11 and the second secondary winding Ns12 are connected in series with each other. The secondary synchronous rectification driver 13 is configured to perform switch operations in synchronization with the primary switch circuit 11, so that the synchronous switch SS can function as a rectifier. With the repeated switch operations of the synchronous switch SS, the high-frequency AC voltage induced across the second secondary winding Ns12 can be rectified into a desired DC voltage. This desired DC voltage will be filtered by an output capacitor Cout so as to generate an output DC voltage Vout which is provided to a load (not shown).
Also, the flyback converter of FIG. 1 further includes a current transformer 14 which is connected in series between the drain terminal of the synchronous switch SS and an output terminal. The current transformer 14 has a primary winding 15 and a secondary winding 16 with a relatively large turn ratio (about 1:100). When the secondary current I2 flows through the primary winding 15, the secondary winding 16 will generate an output signal which is proportional to the current flowing through the primary winding 15. The output signal will be sent to the secondary synchronous rectification driver 13 so that the secondary synchronous rectification driver 13 can drive the synchronous switch SS to initiate the synchronous rectification process when a current with a correct polarity flows through the second secondary winding Ns12 and the synchronous switch SS. The flyback converter of FIG. 1 further includes a feedback control circuit 12 which is connected between an output terminal and the primary switch circuit 11. The feedback control circuit 12 is configured to detect variations on the output voltage of the flyback converter and in response thereto generate an error signal, thereby enabling the primary switch circuit 11 to stabilize the output voltage of the flyback converter at a predetermined level based on the error signal.
As is well known in the art, the current transformer 14 is a magnetic element. When the current transformer 14 is conducting currents, a considerable power supply will be generated due to the heated transformer. The power loss associated with the current transformer includes copper loss, iron loss, eddy current loss and hysteresis loss. These losses would degrade the efficiency of the flyback converter and deteriorate the isolative characteristic of the current transformer 14. Further, the life of the current transformer 14 would be shortened and the overall efficiency of the flyback converter would be limited.